Wedge-top basins of the Ukranian Outer Carpathians and the Northern Apennines as tracers of (almost) coeval evolution of accretionary-collisional orogens

The northeast migration of the Oligocene-Pleistocene foreland basins system of the Northern Apennines brought the formation and younging, in the same direction, of wedge-top basins which were moving on top of the Ligurian prism; these basins allow to reconstruct the evolution and propagation of the accretionary prism and orogen growth. A first comparison of the structural and stratigraphic evolution of Northern Apennines and Ukrainian Outer Carpathians foreland basins system shows they are very similar and allow to reveal the occurrence of wedge top basins in the Ukrainian orogenic wedge. Ukrainian Carpathians, now NW-SE trending, are a thin-skinned thrust belt considered to be the Cretaceous-Neogene accretionary prism formed as a result of south-western subduction of the Carpathian  basin, a portion of the Northern Penninic ocean. Neogene Carpathian Foredeep can be divided into the Inner zone, accreted to frontal Boryslav-Pokuttya nappe, and the Outer zone overlaying the European plate.

In 2023-2024, geological-structural and sedimentological field studies were carried out in this frontal part of Ukrainian Carpathians. In Boryslav-Pokuttya nappe frontal zone we identified Early Miocene (23-16 Ma) wedge top basins. They represent NW-SE trending narrow and steep synclines filled with synorogenic deposits grouped in Polyanytsa, Vorotyshcha and Stebnyk Fms. The Polyanytsa and Vorotyshcha Fms start with thick chaotic complex, which lies on the erosional surface above the Oligocene and Eocene turbidites. The chaotic complex consists of matrix-supported debris-flow deposits containing exotic Paleozoic and Riphean fragments of European platform and Carpathian flysch. Specifically, the Polyanytsa Fm is made of gray flysch-like deposits with olistostrome lenses in the inner zones of the studied wedge-top basins. The shallow-water Vorotyshcha Fm is made of gray clays and sandstones and locally conformably overlays the Polyanytsa Fm and, since 21 Ma, evaporitic lenses mark the inception of Inner Foredeep for the more external portion of the Boryslav-Pokuttya nappe. In both wedge-top and Inner Foredeep basins, the Vorotyshcha Fm is followed by shallow-water deposits of the Stebnyk Fm. At the Early-Middle Miocene transition, the sedimentation in the wedge-top basins above the Boryslav-Pokuttya nappe is completed and chaotic complex were deposited in the Inner Foredeep formed to the NE of the Boryslav-Pokuttya nappe. In fact, since 16 Ma, Boryslav-Pokuttya nappe was accreted to the Outer Carpathian prism and became part of the orogenic wedge while the detachments began to advance within the Inner Foredeep which corresponds to the second wedge-top basin identified. The latter is the Middle-Late Miocene wedge-top basin (16-10 Ma), a gently dipping syncline infilled with shallow-water salt-bearing sediments with a thick chaotic complex at the base. In this wedge-top basin the sedimentation ends by 10 Ma with a conglomerate sequence sourced by the Carpathian Flysch.

These newly revealed wedge-top basins in the Ukranian Carpathians and related major tectonic events are (almost) coeval to the lower Miocene, middle Miocene and late Miocene nappe advancement of the Northern Apennines posing the bases for a better comparison between the two orogens